1. Field of the Invention
The present invention relates to a power operated piston tool with piston automatic return.
2. Brief Description of the Background of the Invention Including Prior Art
In power operated piston tools, of indirect action, designed for driving fastening elements into a base like masonry, concrete, wood or the like, a piston element, driven by firing gases, coming from firing cartridge, is used as an intermediary element, acting upon the fastening element, as opposite to tools of direct action, where firing gases act directly upon the fastening element.
In the tools of indirect action, the piston after fastening is in its forward position, that its piston tip is at the fastener guide muzzle end. In order to return the piston to its firing position, in conventional, semiautomatic tools, manual reloading is required. This reloading consists of pulling backward the barrel means, having a returning tooth, engaging the respective cavity on piston means, which causes the piston to return. This operation is also used to shift the firing cartridge belt by one position, thus making the tool ready for next firing.
In other kind of power piston tools, described in patent EP 0223740, the piston return is actuated by the firing gases, which, after pushing the piston to its forward position, while still expanding, cause the piston to retract. However, after retracting, the piston strikes against its rear abutment, thus causing the reflection effect, which may displace the piston of some distance from its firing position, creating a dead space, decreasing the firing power of a subsequent fire. As a result, fasteners are driven into the base at uneven depth and the firing chamber becomes spoiled with firing remains, due to incomplete burning of blowing charge. Besides, problems can occur when the piston itself is driven into the base after fastening operation. In that case, to return the piston to its firing position, an additional manual push-rod is needed. In any case, after transporting or other periods of prolonged disuse of the tool, it is always useful to check with manual push-rod, whether the piston is well in its firing position, the only one initial position ensuring tool correct functioning.
In another kind of power piston tool, illustrated in U.S. Pat. No. 3,331,546, to return the piston member, a plurality of stacked washers constructed from polyurethane elastomer, are applied. The washers are positioned within the barrel, on the piston shank. After firing the washers are rapidly collapsed, thus absorbing part of firing energy and functioning as a buffer. At the end of power stoke, and after dissipation of the firing gases, escaping through vent holes in the barrel, the washers will return to their original shape, due to elastic memory, and thus they return a piston to its firing position. The venting holes, situated in washers"" side portions cause stress concentration and thus damage to spring elements. The other problem arises when exchanging worn-out washer elements, as they require strictly defined positions and quantity, otherwise malfunction may occur, if not a serious tool damage.
1. Purposes of the Invention
It is an object of this invention to provide a power operated piston tool, free from above mentioned imperfections, in which the piston return is carried-out mechanically, by use of one-piece elastic returning bush.
It is another object of the present invention to provide a power operated piston tool in which a piston is correctly positioned in its initial position after firing.
These and other objects and advantages of the present invention will become evident from the description which follows.
2. Brief Description of the Invention
The present invention provides for a power actuated piston tool for driving fasteners into a base like masonry, concrete, wood. The power actuated piston tool with automatic piston return is composed of an external barrel with a guiding barrel and a guide fastener and a piston means moveably mounted in the guiding barrel and the fastener guide. The piston means is formed of a piston shank inserted in the fastener guide and a piston head guided in the guiding barrel. A returning bush is situated on the piston shank between the piston head and the fastener guide. The returning bush is made of elastomeric material and has a shape of bellows, whose diameters both external and internal are regularly varied. The walls are so formed that the returning bush approximates to a sinusoid, or to a stack of truncated-spherical segments, or frusto-conical segments, or a stack of barrel-shaped segments or/and other revolution segments, creating uniformly spaced swellings and narrowings with a wave-like structure.
The returning bush according to this invention advantageously has a maximum internal diameter of at least one segment at its both ends of a slightly smaller size than the respective diameter of the remaining segments, so the end segment walls are thicker than the other segment walls.
The internal end surface of external segments is markedly curved outside in such a way, that the centre of curvature of the bush ends is clearly distanced from the bush face. The length of the returning bush is selected in such a way that after initial blocking, the piston shank end face does not reach its extreme forward position and remains at a distance from the base, the distance being slightly greater than the head height of the fastening element. The maximum external diameter of the returning bush according to the present invention is smaller than the internal diameter of the guiding barrel, so that, in an initial blocking position of the returning bush, its external diameter still remains smaller than the internal diameter of the guiding bush, thus preserving a slight clearance.
After fully driving the fastening element into the base, and subsequent stopping of the piston""s movement, the energy accumulated in the returning bush is relieved and returns the piston to the rear, due to the shape-memory of the elastomeric material. It is the first and the greatest returning force acting upon the piston. After returning the piston to its rear position, there remains some slight force due to not fully recovering the bush free shape, which force retains the piston in its rear position, i.e. the firing position.
The novel features which are considered as characteristic for the invention are set forth in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.